Rename main branch

[nfahlgren]

Describe your changes
Updates code, documentation, and CI workflows to replace the default branch name with main.

Type of update
Is this a: Update

Associated issues
#588

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #940 (7605728) into main (d1456d0) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #940   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          141       141           
  Lines         6011      6011           
=========================================
  Hits          6011      6011           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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